Now, as represented by the field of terrestrial broadcast, data communication is rapidly being shifted from analog modulation to digital modulation. The digital modulation is roughly divided into three according to a data value to be transmitted: ASK (Amplitude Shift Keying) that controls the amplitude of a carrier wave; FSK; and (Frequency Shift Keying) that controls the frequency of a carrier wave; and PSK (Phase Shift Keying) that controls the phase of a carrier wave. In the above three digital modulations, overlapping between data of “0” and data of “1” in the phase plane becomes smaller in the order of ASK, FSK, and PSK.
Now, assume that, in BASK (Binary Phase Shift Keying), data of “1” is represented by a carrier wave and data of “0” is represented by a carrier obtained by inverting the phase of the carrier wave representing “1” by 180 degrees, and that the phases thereof are switched instantaneously so as not to allow a stationary point of the phase to occur at the time of data switching. In this case, overlapping between “1” and “0” does not exist in the analytic viewpoint. That is, in the above assumption, the probability that a reception signal of “0” is erroneously read out as data of “1” is 0 in the analytic viewpoint, excluding influence of noise mixed in a circuit or transmission lime. Thus, a satisfactory bit error rate can be realized.
FIG. 12 is a graph illustrating, according to the type of the digital modulation method, a relationship between a value obtained by dividing the energy allocated to each bit of a reception signal by a noise power and bit error rate. This is disclosed in NPL 1 to be described later. As is clear from the graph, in the case where the SNR (Signal-to-Noise Ratio) of a reception signal, i.e., a value proportional to that plotted on the horizontal axis is the same between the modulation methods, the PSK exhibits the most satisfactory bit error rate.
By selecting the PSK as the modulation method, it is possible to achieve a desired bit error rate while reducing the required SNR to the smallest possible. Further, in this case, it is possible to increase the estimation tolerance of noise to be mixed in a reception circuit during signal input to signal demodulation. This enables an increase of the tolerance of thermal noise to be set in a low noise amplifier of a wireless reception circuit, with the result that power reduction of a reception circuit can be achieved.
Unlike the ASK and FSK, the PSK requires a reference phase in demodulation. The reason for this will be described below.
In the case of a digital modulation (for example, binary FSK) other than the PSK, a signal whose phase advances from 0 degrees to 90 degrees with respect to a carrier wave and a signal whose phase advances from 90 degrees to 180 degrees have the same digital data. A reception side determines whether the modulation with respect to a carrier wave is modulation in the phase-advance direction or modulation in the phase-delay direction to thereby acquire digital data. Further, in data demodulation on the reception side, the phase of the carrier wave need not be made to coincide with that on the transmission side.
On the other hand, in the case of the PSK, digital data corresponds to the phase shift amount of the carrier wave, so that a signal whose phase is shifted by 45 degrees with respect to the carrier wave and a signal whose phase is shifted by 225 degrees with respect to the carrier wave have different data. Thus, in order to properly reproduce data from a PSK reception signal, it is necessary to have a reference phase for correctly demodulating a phase-shifted reception signal.
In the case of, e.g., a wired communication with low data rate, a clock required for reading out data can be sent through a communication line provided separately from a communication line of the data. Thus, it is comparatively easy to grasp the reference phase on the reception side in the case of a wired communication. However, in the case of a wireless communication, there exists only one physical transmission line, so that it is difficult to simultaneously send the clock required for demodulation and data.
In order to cope with the above difficulty, a configuration is required in which a clock previously prepared on the reception side is made to be phase-synchronized with the carrier wave of a reception signal to find a correct reference phase for use in demodulation. Processing that detects the reference phase in such a manner is referred to as carrier wave recovery. The lower the synchronization accuracy is, the higher the bit error rate becomes. Thus, in order to improve digital communication quality, it is extremely important to develop a technique for realizing highly accurate carrier wave recovery.
In the current wireless communication environment, the ASK or FSK is still mainly used in UHF band, while in ISM band (Industry Science Medical band) of 2.4 GHz, Bluetooth®, ZIGBee®, or the like adopts the PSK. Thus, there is a demand of developing the carrier wave recovery technique also in the application to a weak power band like the ISM band.
For the carrier wave recovery, a feedback loop called Costas loop is used. This feedback loop is implemented in the analog signal region that has been frequency-converted by, e.g., a mixer.
Along with progress of a recent LSI technology and digital signal processing technique, there has been proposed a technique for realizing the feedback loop in the digital signal region after A/D Conversion. PTL 1 and PTL 2 disclose such a technique. In this technique, the phase of a received modulation signal and carrier wave phase in a reception circuit are A/D converted and then multiplied in a complex multiplier to detect the phase difference between them. Then, a loop filter is used to remove signal components other than a desired signal component from the reception signal and, after that, a numerically-controlled oscillator is used to output a synchronization correction signal.